The invention relates to a measuring device of the rms value of a signal, notably for current measurement in a solid-state trip device.
In state-of-the-art solid-state trip devices, two methods are essentially used to measure the rms value of a signal representative of a current.
According to a first method, used essentially in analog trip devices, the peak value of a signal is determined and this value is multiplied by 0.707. Although this method gives satisfactory results when the input signal is a pure sinusoidal signal, it gives rise to very large errors as soon as the input signal is disturbed.
According to a second method, used in microprocessor-based trip devices, the input signal is sampled and the rms value calculated by summing the squares of a certain number of samples. This second method, although it provides more accurate results, is relatively complex to implement and consequently expensive.